In general, undercarriages include an oleo damper comprising a cylinder in which a rod is mounted to slide telescopically. In direct undercarriages, the cylinder constitutes the strut of the undercarriage and the rod carries one or more wheels at its bottom end. In undercarriages using a hinged rocker beam, the wheels are carried by the hinged rocker beam and the shock absorber is mounted to be hinged firstly to the rocker beam and secondly to the aircraft or to a strut of the undercarriage.
The oleo damper includes at least one pneumatic chamber filled with gas that is compressed when the shock absorber is shortened. It also includes means for throttling the hydraulic fluid while it is being shortened, which assumes that there is a supply of hydraulic fluid connected in series with the chamber for the gas. Damping impacts during landing requires the shock absorber to have a certain shortening stroke over which the gas is compressed and the hydraulic fluid throttled. As a general rule, this stroke is of the order of a few tens of centimeters. This stroke and the length of the pneumatic chamber and the length of the supply of hydraulic fluid determine the empty length of the shock absorber.
Aircraft are being optimized more and more, and the room available in the wheel bay for receiving the undercarriage is becoming more and more limited. In general, the aircraft manufacturer specifies the position(s) of the wheel(s) when the undercarriage is in the extended position and also the position(s) of the wheel(s) when the undercarriage is in the retracted position. The designer of the undercarriage then attempts to design an undercarriage having a mechanism that enables the two specified wheel positions to be complied with and that can accommodate a shock absorber of appropriate length. In certain recent projects that are particularly optimized, it has not been possible to make use of an oleo damper of conventional design and that is also capable of satisfying all of the specified constraints. Other than requesting the aircraft manufacturer to provide more volume in the wheel bay, it would therefore appear necessary to provide a shock absorber that is shorter and more compact, while nevertheless allowing for the strokes that are normally used.
The situation becomes even more complicated if, as happens with certain hinged rocker beam undercarriages, such as those of helicopters, the shock absorber is also required, in addition, to drive the extension/retraction function for the undercarriage. The retraction stroke is often longer than the shock-absorbing stroke, thereby lengthening the shock absorber correspondingly.
Documents GB 2 057 629, EP 0 398 797, FR 819 164, GB 942 804, and EP 1 085 233 disclose shock absorbers including at least one external auxiliary cylinder receiving a pneumatic chamber of the shock absorber.